In past decades, the flow of mobile data has multiplied, which has made cellular networks require higher capacity and provide broader coverage. Satisfying indoor usage is extremely important since 70% of data usage is used indoors. However, because serious coverage losses happen when going from outdoors to indoors, using networks provided by appliances, like a conventional macro cell to indoor coverage, will cause obvious problems. Nowadays, there are plenty of solutions for indoor coverage, like distributed antenna systems (DAS), which use cells and fibers. The cells include micro cell technologies, micro cells, and family base stations consisting of e-utran nodes B (also known as evolved node B, eNBs), which are low-cost and use low power. The DAS based on fibers consist of concentrated baseband units (BBUs) and distracted remote radio heads (RRHs). They are connected through fibers, and baseband signals (I/O data) are transmitted between them.
High density network topology is compulsory, no matter eNBs or RRHs is used, in order to ensure indoor coverage and the high capacity. However, base stations of the neighboring cell base stations or RRHs extremely limit and interfere with network performance. Therefore, efficient interference negotiation becomes necessary within the areas which are mutually covered by the neighboring cell base stations and RRHs. The “cell” in the following contexts means areas covered by eNBs or RRHs. Typical technologies of inter-cell interference coordination (ICIC) are fractional frequency reuse (FFR) and soft frequency reuse (SFR). Both methods distinguish the cells into central bands and edge bands. The central band is used as a reuse mode by users in the central zones. The edge band is divided into several sub-bandwidths which are viewed as higher reuse modes utilized in reuse for users in edge zones. In SFR techniques, the entire bandwidth is divided into several sub-bandwidths where one of the sub-bandwidths is assigned to users in edge zones, and other bandwidths are used by users with lower power in the central zones. There are also plenty of developed ICIC techniques, like reversed link inter-cell inference proposed by SHANGHAI BELL. The purpose of this technique is to increase the reuse modes of the frequency resources. This concept assigns different reserved sub-bandwidths to neighboring cells in order to replace an available sub-bandwidth. The reserved sub-bandwidths are usage-limited, but available for usage, for the cells in present services.
In general, Macro Cells are arranged in hexagons in open outdoor areas. For the arrangement of the cells shown in FIG. 1, preferable frequency scheduling for the ICIC utilizes edge bands in a reuse mode called 3 and 1/3, which can be easily completed manually. For indoor coverage, the complex and various structures and materials of buildings results in an irregular arrangement of cells. The number of cells and key interference are varied. Please refer to FIG. 2, which illustrates an example of an arrangement of a student flat having two floors. Cell 7 only has one neighboring cell, while cell 5 has five neighboring cells. Therefore, the outdoor frequency 1/3 arrangement mode cannot be directly utilized. In addition, the manual scheming frequency method is nearly impossible to achieve because the number and disposed location of the cell base stations or RRHs are unpredictable.